Equipment information

This instruction manual is for use with the 885A series Differential DC Voltmeters. This instrument is available as either a line-powered instrument (Model 885A), or as a combination line-powered/battery-powered instrument (Model 885AB). Both instruments are half-rack size and are equipped with resilient feet and tilt-up bail for field or bench use. A single instrument may be mounted in a standard 19 inch rack with metal handle-rack adapter kit 881A-102. A single instrument may be mounted side-by-side with another Fluke half-rack size instrument by using metal handle-rack adapter kit 881A-103. 1-3. The 885A series instruments can be used as: (a) Conventional voltmeters for rapid determination of dc voltages from 0 to 1100 volts dc to within ±3% of range setting, (b) Differential voltmeters for precise measurement of dc voltages from 0 to 1100 volts to an accuracy of ±(0. 0025% of input + 0. 0001% of range + 5 uv), and (c) As megohmmeters for measurement of resistance from 10 megohms to 111, 000 megohms with a typical accuracy of 5%. The instrument can also be used to measure voltage excursions about a nominal value. One feature which should be emphasized is that no current is drawn from the unknown, when the voltmeter is at null, at up to 11 volts. Thus the determination of the unknown potential is independent of its source resistance. Above 11 volts, the input resistance is 10 megohms. 1-4. To minimize errors due to common mode voltages, the 885A series is provided with exceptionally high leakage resistance to ground, typically several hundred thousand megohms. Where ground loop current may be present, complete isolation from the power line through battery operation of the 885AB entirely eliminates this source of error. The 885A series contains a polarity switch for equal convenience in measuring positive or negative voltages and an adjustable recorder output which makes the instrument particularly useful for monitoring the stability of dc voltages. 1-5. When used as a differential voltmeter, the 885A and 885AB operates on the potentiometric principle. An unknown voltage is measured by comparing it to a known adjustable voltage with the aid of a null detector. An accurate standard for measurement is obtained from an 11 volt reference supply derived from a pair of temperature-compensated zener diodes. The known adjustable reference voltage is provided by a Kelvin-Varley voltage divider with four decades of Fluke precision wirewound resistors and a high-resolution interpolaring vernier that are set accurately by five voltage readout dials to give a six digit readout. The unknown voltage is indicated by the voltage readout dials. For voltages between 11 and 110 volts, an input attenuator divides the unknown voltage by 10 before it is measured potentiom-etrically; between 110 and 1100 volts, the input attenuator divides the unknown voltage by 100.

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